Black brane fluid flows
We study how the hydrodynamics of a black brane changes when we enclose it in a finite cylindrical cavity with Dirichlet boundary conditions. For neutral black branes, we see how the Gregory-Laflamme instability switches off and study the critical point where the instability disappears. For black D3 branes, as the cavity radius decreases one obtains the sequence of descriptions "blackfolds > fluid/AdS-gravity > membrane paradigm". We show that, to leading order above extremality, "sound comes from the throat".